Re. Step 1: We start by believing in the bare minimum : our own subjective experience exists.

the reduction to the pure ego and to the solipsistic sphere of the mine, in the sense that Husserl understands it, is forbidden by the very structure of Dasein, that an ego without world and without others is never given to us, and that my relation to the other is not established by an Einfühlung [empathy] bridging two subjectivities but has always already come about.

So the problem with step 1 is that the premise, an isolated ego, is a misconception. For Dasein, the ego — the I — is one thing amongst others: the I, the you, the table. Each thing is managed according to what it is but at no stage is there only an I.

Quote is from Jacques Derrida's Heidegger: The Question of Being and History - pages 120-121

Re. Step 1: We start by believing in the bare minimum : our own subjective experience exists.

the reduction to the pure ego and to the solipsistic sphere of the mine, in the sense that Husserl understands it, is forbidden by the very structure of Dasein, that an ego without world and without others is never given to us, and that my relation to the other is not established by an Einfühlung [empathy] bridging two subjectivities but has always already come about.

So the problem with step 1 is that the premise, an isolated ego, is a misconception. For Dasein, the ego — the I — is one thing amongst others: the I, the you, the table. Each thing is managed according to what it is but at no stage is there only an I.

Quote is from Jacques Derrida's Heidegger: The Question of Being and History - pages 120-121

Continuation

So, in the end, we have two fundamental aspects of reality: 1. Subjective experience and qualia which we know to exist since the beginning. 2. The objective mathematical reality, whose existence we have inferred using tiny leaps of faith.

Re 1. Experience is being-in-the-world from the beginning, amongst all the phenomena at hand. The subject owns the experience; the self/not-self distinction develops gradually, understanding of phenomena is refined. Discovery of things in-the-world like mathematical constants are still in-the-world. (That was re 2.)

Re. Step 1: We start by believing in the bare minimum : our own subjective experience exists.

the reduction to the pure ego and to the solipsistic sphere of the mine, in the sense that Husserl understands it, is forbidden by the very structure of Dasein, that an ego without world and without others is never given to us, and that my relation to the other is not established by an Einfühlung [empathy] bridging two subjectivities but has always already come about.

So the problem with step 1 is that the premise, an isolated ego, is a misconception. In one's thought, the ego — the I — is one thing amongst others: the I, the you, the table. Each thing is managed according to what it is but at no stage is there only an I.

Quote is from Jacques Derrida's Heidegger: The Question of Being and History - pages 120-121

Re. Step 1: We start by believing in the bare minimum : our own subjective experience exists.

the reduction to the pure ego and to the solipsistic sphere of the mine, in the sense that Husserl understands it, is forbidden by the very structure of Dasein, that an ego without world and without others is never given to us, and that my relation to the other is not established by an Einfühlung [empathy] bridging two subjectivities but has always already come about.

So the problem with step 1 is that the premise, an isolated ego, is a misconception. For Dasein, the ego — the I — is one thing amongst others: the I, the you, the table. Each thing is managed according to what it is but at no stage is there only an I.

Quote is from Jacques Derrida's Heidegger: The Question of Being and History - pages 120-121